A system in common use for detecting the velocity of fluid-born vehicles, such as aircraft and ships, is the so-called Doppler navigation system. In such systems for aircraft speed measurement, a very narrow radio frequency beam is projected forward of the aircraft at an angle of perhaps 20.degree. to the vertical. The aircraft then captures an echo return and senses the average Doppler-caused frequency shift to obtain an indication of the rate at which the aircraft is approaching the area causing the echo. A trigonometric function of the effective beam angle is used to convert the indicated rate of closure between the aircraft and the area under observation into the desired horizontal velocity component.
In a Doppler system, the beam must be narrow to achieve accuracy. This requires that a highly directional transmitting antenna must be employed, leading to substantial difficulty in making the antenna installation small enough and convenient enough for use in high speed aircraft. Even with a restricted beam width, Doppler systems contain large errors arising from changes in the reflectivity of the surface under observation. Since the beam employed in a Doppler system must have finite width, the speed measurement is actually taken from a relatively large observed area. If the area under observation is a smooth surface, then the amount of energy reflected from the nearer portions of the smooth surface will produce a stronger echo return, at which the angle of incidence is more favorable to a strong echo return than at the more remote portions of the area under observation, in which portions the angle of incidence is less favorable to a strong echo return. On the other hand, when the beam falls on irregular shapes, such as trees in a forest, slight changes in the vertical angle make little difference in the average angle of incidence, and more nearly equal weight will be accorded to the near and far portions of the beam. This difference in reflectivity can introduce a substantial error into a Doppler system and greatly reduces its accuracy. Furthermore, the utility of a Doppler system when used over very smooth surfaces, such as calm water, is greatly diminished, if not totally nullified, due to the lack of ground scatterers capable of reflecting the forward-projected beam in the direction of the aircraft. Furthermore, there are many applications, particularly military, in which the required forward projection of wave energy in advance of the aircraft is highly undesirable.
The applicants' system overcomes the above-mentioned disadvantages of the Doppler system. Instead of projecting a narrow beam forward of the vehicle, the present system requires only a broad beam of pulse energy transmitted substantially directly to the reference surface, i.e., in the case of an aircraft or ship, vertically down to the ground or sea bottom. Operation with a broad beam width enables the use of a very small and compact antenna. Furthermore, because the transmitted power is confined essentially along the vertical, the sensitivity of the system is not affected by the specific characteristics of the reflection surface and the applicants' system is highly effective over smooth water. Still further, since, unlike the Doppler system, applicants' system does not rely on accurate detection of the frequency or phase content of the reflected energy, frequency-altering phenomena in the medium through which the wave energy propagates do not disturb the operation of the system.
A system operating on the same basic principles as that of applicants' is disclosed in U.S. Pat. No. 3,147,477 entitled "Speed Measuring System" issued in Sept., 1964 to F. R. Dickey, Jr. The system disclosed herein is a refined and simplified version of the Dickey system.